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rallas
10-02-2016, 04:35 PM
We are running Bitzer CSH9571 screw compressors on our 280 ton R134a chiller. We using their infinite load control strategy where the loading and unloading solenoids are pulsed to increase or decrease load. Even when no load or unload pulses are given we see the compressor load up, indicated by current increase and suction pressures dropping slightly. This continues until the evaporator outlet temperature falls below setpoint and the logic fires the unloading solenoid to reduce capacity. This unloads the compressor and we see reduced current, reduced cooling capacity and suction pressure goes up a little. The cycle then starts over again.

We have replaced the loading solenoid valve, thinking that the valve might be leaking and allowing oil pressure past the seat and allowing the slider valve piston to increase load slowly. No change was observed with a new loading solenoid valve in place.

We used to have significant liquid flood back issues that were believed to contribute to the automatic loading due to liquid refrigerant expanding in the slider valve piston chamber causing compressor loading. We have since installed much better electronic expansion valves controlling to much higher superheat and we are 100% no liquid is being returned to the compressor. This did not seem to impact the uncontrolled loading at all.

We have checked to make sure that the loading solenoid valves do not have any residual voltage on the coils or unintended PLC control actuation that could cause the valve to be open when it shouldn’t or lift it off the seat enough to allow for leakby. We even removed all the solenoid coils and observed no change. The compressor would still load from a fully unloaded state to a fully loaded state with no control input.

The auto loading behavior is less severe at lower loads (<50%) but gets pretty aggressive as you load the compressor above 70%.

What else could cause a compressor to automatically load up like this?

RANGER1
10-02-2016, 09:31 PM
https://www.bitzer.de/shared_media/documentation/sh-170-4.pdf

I would suggest unload/load piston seal is worn or cylinder scored.
Discharge pressure would push on end of slide valve forcing oil past the seal, loading compressor, as long as Y4 solenoid is not leaking.

GHAZ
10-02-2016, 09:50 PM
It's possible the gasket on the end of the piston chamber is split or damage, because it's sits in the oil separator side and the discharge pressure could be clipping in

rallas
10-02-2016, 09:59 PM
https://www.bitzer.de/shared_media/documentation/sh-170-4.pdf

I would suggest unload/load piston seal is worn or cylinder scored.
Discharge pressure would push on end of slide valve forcing oil past the seal, loading compressor, as long as Y4 solenoid is not leaking.

What would cause a scored cylinder?

We are looking at slider piston seals and chamber gasket leaks as potential failures that would allow higher pressure on the oil side of the slider piston, causing the auto loading. Have not been able to prove it, but should be tearing into two compressors soon to look for evidence. Strange thing is that we see it in 2 of the 4 compressors that we have operating. The come from two seperate chiller skids, only one per skid is acting up real bad.

We are also trying to understand what would cause these gaskets or seals to fail.

RANGER1
11-02-2016, 12:31 AM
What would cause a scored cylinder?

We are looking at slider piston seals and chamber gasket leaks as potential failures that would allow higher pressure on the oil side of the slider piston, causing the auto loading. Have not been able to prove it, but should be tearing into two compressors soon to look for evidence. Strange thing is that we see it in 2 of the 4 compressors that we have operating. The come from two seperate chiller skids, only one per skid is acting up real bad.

We are also trying to understand what would cause these gaskets or seals to fail.

There could be a number or reasons of why you have these problems
Scoring could happen, reasons could be mechanical damage with metal on metal because of worn out seals or worn out slide valve.
You could possibly check unloader system out by isolating & depressurising compressor & unload cylinder.
Using nitrogen to simulate things to narrow down problem, energising/de-energising solenoids, observing leakage through solenoids etc.
This may help determining next step, like dis-assembly of components.

Saul
25-02-2016, 10:56 AM
Hi Ghaz
total success in changing the motor.
thank you.

srinitkr
25-02-2016, 04:45 PM
i have same problem in york screw chiller ycas model

rallas
29-02-2016, 04:36 PM
i have same problem in york screw chiller ycas model

Have you been able to fix the uncontrolled loading or do anything to live with this behavior?